Vitamin D is responsible for the expression of about 1,000 genes, including a number of genes linked to things like MS, Crohn’s disease, systemic lupus, rheumatoid arthritis, and cancer. It’s an essential vitamin that acts like a hormone, but a whopping 40 to 75% of Americans aren’t getting enough.

In addition to its many important roles, microbiome specialist Dr. Steven Gundry believes vitamin D can heal autoimmune disorders. Dr. Gundry discovered that autoimmune patients were almost always vitamin D deficient, and prescribes high levels to heal the disorders through repairing the gut.

“I am convinced that all autoimmune disease begins and ends in the gut,” Dr. Gundry told Goop. “Our gut wall is the same surface area as a tennis court! And the lining of our gut is only one cell thick. These cells are all locked arm in arm (like the kid’s game of Red Rover). Once that gut wall is penetrated by lectins in our food, NSAIDs like Ibuprofen or Naprosyn, or more than a couple of glasses of wine, it is the job of gut stem cells to rapidly grow and seal the gaps. But these stem cells need vitamin D to grow.”

“The patients who come to see me with autoimmune diseases who haven’t gotten better after going gluten-free or following other like-minded advice from their practitioners, all have very low levels of vitamin D. And most of them have to take large amounts of the supplement to get their levels to budge into the normal range.”

While your body can make vitamin D from the sun, anything from sunscreen to pollution to living in places with less sun exposure during certain times of the year may cause a deficiency. It’s not easily obtained from food sources, so many doctors recommend supplementation. While experts agree up to 4,000 IU/day is safe, we suggest discussing dosage with your natural healthcare practitioner.